In the past three decades, electricity has risen from 25% to 40% of end-use energy consumption in the United States. With this rising demand for power comes an increasingly critical requirement for highly reliable, high quality power. As power demands continue to grow, older urban electric power systems in particular are being pushed to the limit of performance, requiring new solutions.
Wire forms the basic building block of the world's electric power system, including transformers, transmission and distribution systems, and motors. The discovery of revolutionary high-temperature superconductor (HTS) compounds in 1986 led to the development of a radically new type of wire for the power industry; this discovery is the most fundamental advance in wire teleology in more than a century.
HTS wire offers best-in-class performance, carrying over one hundred times more current than do conventional copper and aluminum conductors of the same physical dimension. The superior power density of HTS wire will enable a new generation of power industry technologies. It offers major size, weight, and efficiency benefits. HTS technologies will drive down costs and increase the capacity and reliability of electric power systems in a variety of ways. For example, HTS wire is capable of transmitting two to five times more power through existing rights of way.
This new cable will offer a powerful tool to improve the performance of power grids while reducing their environmental footprints. However, to date only short lengths of coated conductor wire samples have been fabricated at high performance levels with any of the conventional fabrication processes.
In order for HTS technology to become commercially viable for use in the power generation and distribution industry, it will be necessary to develop techniques for continuous, high-throughput production of HTS tape. Several challenges must be overcome in order to enable the cost-effective production of long lengths (i.e., several kilometers) of HTS-coated conductor wire.
Vapor deposition is a process for manufacturing HTS tape in which vapors of superconducting material such as YBCO are deposited on a tape-like length of buffered metal substrate, thereby forming an HTS coating on the tape substrate. Well-known vapor deposition processes include physical vapor deposition (PVD), chemical vapor deposition (CVD)), and pulsed laser deposition (PLD). PLD has shown great promise for the deposition of superconducting thin films, due in large part to its operational simplicity, its flexibility in vacuum requirements, and the congruent, stoichiometric transfer of material that results from the generation of a highly forward-directed plume from target to substrate.
In a pulsed laser deposition (PLD) process in which a laser is used to evaporate a material, where atoms of the material subsequently coat a surface that is exposed to the evaporated material, thereby forming a film on that surface. PLD is a process suitable for manufacturing HTS wires with high current-carrying capacity In this case, a target comprising a stoichiometric chemical composition of the desired layer is ablated by a pulsing laser, forming a plume of ablated material to which a buffered substrate is exposed, thereby coating the buffered substrate with the desired material and forming a coated wire or tape. Using a PLD process it is possible to deposit a superconducting layer atop a translating flexible buffered polycrystalline metal tape in a continuous, assembly line manufacturing process. However, to date only short lengths of coated conductor wire samples have been fabricated at high performance levels using prior art vapor deposition processes and equipment.
The manufacture of long lengths of HTS tapes via a PLD process necessitates a system that provides for the translation of the tapes through a deposition chamber where they receive the desired thin film coating. Youm, U.S. Pat. No. 6,147,033, dated Nov. 14, 2000, and entitled “Apparatus And Method For Forming A Film On A Tape Substrate,” provides a tape transport system particularly well suited for translating a substrate tape through a deposition chamber.
As described by Youm, the superconducting film is deposited on the tape substrate wound around a cylindrical substrate holder inserted in an auxiliary chamber housed completely within a main deposition chamber. The cylindrical substrate holder rotates during the whole deposition process. Vapors of film materials are supplied form the main chamber through an opening between the two chambers. According to Youm, it is possible to form HTS film rapidly onto a tape substrate having a length up to 300 meters. While this represents a step toward the large-scale production of HTS coated tape, it is limited in its scalability. To achieve significantly longer lengths of HTS coated tape the cylindrical substrate holder must increase in size accordingly, making it impractical to be housed within the main vapor deposition chamber. Thus, a drawback of the vapor deposition process described in Youm is that the system is not easily scalable to produce long lengths (e.g., several kilometers) of HTS coated tape and is therefore not suited for the large-scale production of HTS coated wire.
Several other challenges must be overcome in order to enable the cost-effective production of long lengths (i.e., several kilometers) of HTS coated conductor wire.
A first challenge to the continuous deposition of HTS tapes utilizing a reel-to-reel tape transport system that is not overcome by Youm is the maintenance optimum tape tension throughout the extended deposition runs necessary to high-throughput systems. If the correct level of tautness is not maintained, the tape sags. This results in a variation in the target-to-substrate distance and a compromise of the thin film uniformity.
A second technical challenge to the continuous deposition of HTS tapes utilizing a reel-to-reel tape transport system that is not overcome by Youm is the maintenance of the tape at the optimal speed throughout extended deposition runs. As the spools rotate and the tape is translated through a chamber, the tape must remain at the same position within the deposition zone, regardless of the radii of tape housed on each spool. Lateral, as well as longitudinal, movement of the tape results in an inconsistent and non-uniform deposition resulting in variations in film thickness. The importance of film uniformity cannot be an overemphasized: if there is an insufficient superconducting quality at a single point over the entire length of a few hundred meters of tape, the current carrying capacity of the entire length of tape is compromised. Further, any elements that serve to position the tape must do so in such a way as to not induce stress or strain in the tape, which may damage the delicate thin films
Another technical challenge not overcome by Youm is how to wind the tape onto a spool subsequent to its undergoing the deposition process without damaging the delicate thin film housed thereon. The ceramic grains of superconducting films may fracture if bent beyond a certain strain, which may result in a decrease in the critical current-carrying capacity of the finished superconductor tape.
To achieve the proper bonding of the evaporated material to the substrate during a typical PVD, CVD, or PLD process it is necessary to heat the substrate. Thus, a substrate heater that is capable of sustaining the substrate at a process temperature ranging typically from 500 to 1500° C. is required Current PVD, CVD, or PLD processes typically employ a stationary substrate mounted on a stationary substrate holder, where the substrate holder incorporates a heating element. Since the substrate is in direct contact with the heated substrate holder, heating of the substrate takes place by conduction.
An example of a conventional stationary substrate heater is disclosed in Chen et al., U.S. Pat. No. 6,066,836, dated May 23, 2000 and entitled “High temperature resistive heater for a process chamber”. Chen et al. describes a structure for a processing apparatus such as a chemical vapor deposition chamber that includes a resistively heated substrate holder including a support surface that includes an additional resistive heating element. The heated substrate holder is disk-shaped to accommodate a substrate, such as a wafer, in a semiconductor application. Chen's substrate heater includes a heating element that provides a single heating zone, that is, one uniform temperature is maintained across the entire substrate
However, in the case of a continuously translating substrate as is necessary for a continuous flow manufacturing process, it is difficult to maintain a uniform temperature profile using resistive heaters as disclosed by the prior art, Any local loss of contact with the heating element by a rapidly moving substrate can cause large temperature variations and in turn inhomogeneities in the coating film. Consequently, a technical challenge to overcome is how to heat a rapidly moving substrate in a continuous flow high-throughput manufacturing process for producing long lengths of HTS-coated wire.
In the case of a translating substrate in a continuous flow manufacturing process, multiple temperature zones having different temperature requirements, such as a preheating zone, a deposition zone, and a cooling zone, are desirable. Current substrate heaters do not provide multiple heating zones with differing temperature ranges as required for continuous flow manufacturing of HTS-coated wire and thus are not suited for use in the large-scale production of HTS-coated wire,
In the PLD process, a film is deposited on a substrate by the action of a laser beam impinging on a target material that is located in close proximity to the substrate, thereby creating a plume of ablated material (plasma) to which the substrate is exposed. Conventional PLD systems utilize a single laser beam that impinges on a target mounted on a target manipulator. The target manipulator provides an appropriate target rotation and oscillation. In a particular well-known example, multi-target manipulators may hold multiple targets for sequential use in a PLD process. In this case, as the material of any given target is consumed during the PLD process, the multi-target manipulator indexes from one target to the next. However, in the large-scale continuous production of HTS-coated wire, a multi-laser beam PLD process, in which multiple laser beams impinge on multiple targets simultaneously, may be used, thereby simultaneously creating multiple overlapping plumes to which a translating substrate is exposed. In this way, the deposition region is expanded in length, thereby improving the overall throughput of the PLD process compared with a single laser/single target PLD process. Conventional target manipulators are therefore of limited use in a multilaser beam PLD application.
An example of a conventional target manipulator is described in Kim et al., U.S. Pat. No. 5,942,040, entitled “Multi-Target Manipulator For Pulsed Laser Deposition Apparatus.” Kim et al. discloses a multi-target manipulator for a pulsed laser deposition apparatus, including a driving mechanism that includes a stepping motor and a motion feed for providing rotation to the target disk driving shaft and the target driving motor shaft. The driving mechanism further includes a driving transmission and head-supporting member that transmits a rotational motion for rotating the target disk and the target so as to locate a target material on the focal point of the laser beam.
Although Kim et al provides a multi-target manipulator, the multiple targets are arranged on a circular disk with the intent of being indexed from one to another for consumption one at a time. Although it is conceivable that multiple lasers could be focused on all targets simultaneously, it is not practical for a continuous flow application in which a substrate tape is translating in a straight line, thereby requiring the targets to be arranged in a straight line. A further limitation is that Kim et al.'s the multi-target manipulator provides rotation to only one target at a time. This type of multi-target manipulator is therefore not suited for use in the large-scale production of HTS-coated wire utilizing a continuously translating substrate through a deposition chamber.
It is conceivable that several target manipulators, such as Kim et al.'s multi-target manipulator, could be used in combination with multiple laser beams arranged sequentially in a straight line along the path of the translating substrate tape. However, using such an arrangement of several conventional target manipulators in a multi-laser beam PLD system is very costly and therefore not practical. Also, conventional target manipulators occupy lot of space and as a result, there will be large gaps between targets. This will result in large gaps between plumes from the targets when used with multiple lasers. Consequently, this arrangement of several conventional target manipulators is not economically or practically suited for use in the large-scale production of HTS-coated wire.
It is therefore an object of the invention to provide a tape transport system well suited to the continuous high-throughput manufacture of HTS tapes.
It is another object of the invention to provide a tape transport system that maintains optimum tape tension throughout extended deposition runs.
It is yet another object of the invention to provide a tape transport system that maintains tapes at an optimal target-to-substrate distance throughout extended deposition runs.
It is yet another object of the invention to provide a tape transport system that prevents damage to the newly deposited superconducting films as the tape winds onto a take-up spool.
It is an object of the invention to provide a substrate heater for use with a non-stationary substrate in a continuous flow vapor deposition process.
It is another object of the invention to provide a substrate heater with multiple independent heating zones for use with a non-stationary substrate in a continuous flow vapor deposition process.
It is yet another object of the invention to provide a substrate heater that achieves the desired heating of a translating substrate by a combination of conductive and radiative heating during a continuous flow vapor deposition process.
It is an object of the invention to provide a multi-target manipulator that provides multiple targets arranged in line for simultaneous use in a multi-laser beam PLD process for the large-scale production of HTS-coated wire.
It is yet another object of the invention to cost-effectively provide a multi-target manipulator for use in a multi-laser beam PLD process for the large-scale production of HTS-coated wire.
It is an object of the present invention to provide a PLD apparatus and method for forming highly uniform HTS film on a tape substrate.